The present invention relates broadly to operational amplifiers and in particular to an inductive load driver apparatus with fast switching capability.
In the prior art, operational amplifiers with push-pull output stages and a sensing resistor feedback configuration are used quite extensively in electronic design as current amplifiers. Generally, such operational amplifiers comprise an I.C. operational amplifier, a push-pull output stage and a precision type sensing resistor. The sensing resistor voltage Es is controlled by the magnitude of the input voltage Ein and the closed loop gain of the network. With the load placed in series with the sensing resistor Rx, the standard voltage amplifier has been converted to a current amplifier with the current in the load directly proportional to the voltage developed across the sensing resistor. The output current in this circuit is given by the relationship I.sub.o = Ein A/Rs where A is the magnitude of the closed loop gain of the amplifier and Rs is the ohmic value of the sensing resistor. If precision type resistors are used for those components whose ohmic values enter into the above equation, then an accurate current amplifier will be obtained.
The prior art circuit works quite well as long as the load is purely resistive and no major limitations are encountered. However, when driving an inductive load, there are other factors to consider which makes the task more difficult. One of the primary limitations is how fast can the current through the load be changed. The basic limitation comes from the Ldi/dt of the coil, or more specifically the magnitude of the power supplies in the output stage. The most obvious solution to this problem is to use higher voltage power supplies. However, this soon gets out of hand due to the added stress on the components and the power that is wasted when the circuit is operated in a steady state. The basic problems involved in driving inductive loads can be summarized as follows:
1. Increase complexity in driver and power supply design.
2. Highly stressed components cause a decrease in reliability and an increase in total system cost.
3. Means of dissipating power to maintain components at a safe operating temperature.
4. Waste in the power consumption because the high voltage supplies are only needed for fast switching capability. The present invention provides an inductive load driver apparatus which is capable of operating the circuit from lower values supplies in the steady state, and switching to the higher values supplies during the times when the fast switching capability is needed.